Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 713−722
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Efficient Removal of Polycyclic Aromatic Hydrocarbons, Dyes, and Heavy Metal Ions by a Homopolymer Vesicle Hui Sun,† Jinhui Jiang,† Yufen Xiao,† and Jianzhong Du*,†,‡ †
Department of Polymeric Materials, School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China ‡ Shanghai Tenth People’s Hospital, Tongji University School of Medicine, Shanghai 200072, China
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S Supporting Information *
ABSTRACT: It is an important challenge to effectively remove environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs), dyes, and heavy metal ions at a low cost. Herein, we present a multifunctional homopolymer vesicle selfassembled from a scalable homopolymer, poly(amic acid) (PAA), at room temperature. The vesicle can efficiently eliminate PAHs, cationic dyes, and heavy metal ions from water based on π−π stacking, hydrophobic effect, and electrostatic interactions with the pollutants. The residual concentrations of PAHs, cationic dyes, and heavy metal ions (such as Ni2+) in water are lower than 0.60 and 0.30 parts per billion (ppb) and 0.095 parts per million (ppm), respectively, representing a promising adsorbent for water remediation. Furthermore, precious metal ions such as Ag+ can be recovered into silver nanoparticles by in situ reduction on the membrane of PAA vesicles to form a silver nanoparticle/vesicle composite (Ag@vesicle) that can effectively catalyze the reduction of toxic pollutants such as aromatic nitro-compounds and be recycled for more than ten times. KEYWORDS: homopolymer vesicle, water purification, PAHs, cationic dyes, heavy metal ions
1. INTRODUCTION Water pollution is a worldwide problem that threatens millions of people.1−3 Polycyclic aromatic hydrocarbons (PAHs), phenols, dyes, and heavy metal ions in industrial wastewater seriously influence the quality of fresh water, thus leading to a high risk of illness, disease, and death of human beings, animals, and plants.1,4 More seriously, some pollutants such as PAHs and heavy metal ions have a long-term threat to the water, soils, and creatures due to the easy transfer and bioaccumulation.5,6 There are several approaches to decontaminate polluted water, including solvent extraction, precipitation, adsorption, and ion exchange, etc.7−9 However, one method usually cannot deal with different pollutants. For example, some lifelong organic pollutants such as PAHs and nonionic dyes are difficult to be removed from water by traditional water purification techniques.10−13 Biological and chemical techniques have been used to accelerate the degradation of PAHs. Nevertheless, the bioremediation method lacks proper bacteria and environmental conditions for the growth of bacteria, such as nutrients, pH, gas atmosphere, and temperature, etc. Besides, different strains are needed because of different physiochemical characteristics of PAHs.14,15 The chemical method is widely used but accompanied by heat production, toxic byproducts, and the introduction of oxidants.16−18 Heavy metal ion contaminants such as Pb2+, Zn2+, Cu2+, and Ni2+ have been widely produced by mining, steel, and electroplating industries. Both chemical and physical methods have been applied to remove these contaminants.5,13,19−23 © 2017 American Chemical Society
Chemical precipitation is widely used with an efficiency as high as 99%.7,24 However, this method only works when their concentrations are pretty high and are accompanied by a high cost and secondary pollutions such as toxic foams. Electrochemical methods for reducing metal ions into zerovalent metals are quite efficient.5,25,26 Unfortunately, the cost is very high, and only limited kinds of heavy metal ions can be effectively recovered. Ion-exchange resins and adsorbents with strong negative charges are good candidates for the removal of heavy metal ions from polluted water but may introduce hazardous substances and have similar disadvantages to the above-mentioned methods.13,23,27 Functional nano-objects like micelles,9,28 vesicles,29 magnetic nanoparticles,27,30−33 and silica nanoparticles,13,23,34 are widely used to capture both organic and inorganic pollutants from wastewater. For example, we coated silica nanoparticles with poly(ethylene oxide), an azobenzene derivative, and branched polyethylenimine to adsorb PAHs and anionic dyes from water.34 Gibson et al. used ion-supported silica nanoparticles as an adsorption platform for the removal of arsenate with a capacity of 69 mg/g. Binnemans and co-workers separated heavy rare-earth ions from aqueous solutions with a capacity as high as 100−400 mg/g by EDTA-functionalized magnetic nanoparticles.23,27 Tortora et al. used a sodium-dodecyl-sulfateReceived: October 19, 2017 Accepted: December 6, 2017 Published: December 6, 2017 713
DOI: 10.1021/acsami.7b15242 ACS Appl. Mater. Interfaces 2018, 10, 713−722
Research Article
ACS Applied Materials & Interfaces
vesicles are excellent supporters for immobilizing silver nanoparticles which can be used as a highly efficient recyclable catalyst for the reduction of highly toxic nitro-compounds to less toxic amino compounds.
based micelle to enhance the ultrafiltration of heavy metal ions from water with an efficiency of more than 88%.35 Very recently, polymer vesicles were applied to adsorb PAHs from water with a residual concentration below ppb level by our group.29,36 However, most of the adsorbents cannot meet the demand for simultaneous removal of various pollutants coexisting in the polluted water. Herein, we report a cheap and scalable multifunctional vesicle based on a poly(amic acid) (PAA) homopolymer for effective water remediation. The synthetic process of the homopolymer is simple, energy-saving, and fast, which can be completed within 2 h at room temperature, and the vesicle could be obtained by simply adding water to the polymer solution without purification. As shown in Scheme 1, the PAA
2. MATERIALS AND METHODS 2.1. Materials. Pyromellitic dianhydride (PMDA, 99%), 4,4′oxydianiline (ODA, 98%), silver nitrate (AgNO3, 99.8%), p-nitrophenol (p-NP, 98%), sodium citrate (99%), phosphotungstic acid (PTA, AR), magnesium sulfate anhydrous (MgSO 4 , 99.5%), naphthalene (98%), anthracene (96%), fluoranthene (98%), pyrene (97%), Victoria blue B (80%), methylene blue (70%), crystal violet (90%), nickel sulfate hexahydrate (NiSO4·6H2O, 99.9%), copper sulfate pentahydrate (CuSO4·5H2O, 99.99%), zinc chloride (ZnCl2, 99.95%), and lead nitrate (Pb(NO3)2, 99%) were purchased from Aladdin Chemistry, Co. Ltd. and used as received. DMSO-d6 was purchased from J&K Scientific Ltd. Sodium borohydride (NaBH4, 96%) and other reagents and solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification. 2.2. Synthesis of Poly(amic acid). The synthesis and selfassembly of PAA homopolymer followed our previous work.37 Briefly, PDMA (0.023 mol) dispersed in DMF was added to the DMF solution of ODA (0.023 mol) batch-by-batch within 1 h, and the mixture was stirred for another hour to complete the reaction. 2.3. Self-Assembly of PAA Homopolymer into Vesicles. Two methods were used to prepare PAA vesicles by self-assembly. Method One: The purified PAA homopolymer was dissolved in DMF with a concentration of 2.0 mg/mL, and then deionized water was added dropwise to induce the formation of PAA vesicles. DMF was removed by dialysis against water. Method Two: Water was directly added into the unpurified polymer solution to simplify the self-assembly process. After the polymerization reaction, the PAA homopolymer solution was diluted to different concentrations of 1.0, 2.0, 3.0, 4.0, 6.0, and 8.0 mg/ mL without purification, followed by adding deionized (DI) water dropwise to self-assemble into vesicles. The volume of water added was twice that of the polymer solution. Finally, the PAA vesicle solution was dialyzed in water to remove DMF. The size of the PAA vesicles can be controlled by the initial concentrations of PAA polymer in DMF. For Method One, the PAA homopolymer was purified by precipitating in acetone three times, so some polymer chains with low molecular weight were removed. Subsequently, the sizes of vesicles self-assembled from purified and unpurified PAA were different (e.g., 200 and 300 nm for Methods One and Two at 2.0 mg/mL). However, for Method Two, the self-assembly procedure is much simpler than Method One, and the diameters of the PAA vesicles can be controlled from 261 to 714 nm. Therefore, PAA vesicles were prepared by Method Two in this work. 2.4. Stability Study of PAA Vesicle at Different pH by Dynamic Light Scattering. The diameters and polydispersities (PDs) of PAA vesicles in water at different pHs were measured by dynamic light scattering (DLS). 2.5. Stability Study of PAA Vesicles at Different Ionic Strength by DLS. Aqueous MgSO4 solution was added into the PAA vesicle solution to investigate the ionic responsiveness. The final concentrations of MgSO4 in the PAA vesicle solutions were 0, 0.05, 0.1, 0.2, 0.3, 0.5, 1.0, and 1.5 M, respectively. After equilibrated for 30 min, the diameters and PDs of PAA vesicles were measured by DLS. 2.6. Adsorption of PAHs from Polluted Water by PAA Vesicles. Typically, 1.0 mL of stock solution of PAHs, various volumes of PAA vesicle solutions, and DI water were mixed in a quartz cuvette to give certain concentrations of vesicles (the final volume was 2.0 mL). After different equilibrium time, the fluorescence quenching process was recorded via fluorescence spectroscopy, and the UV absorbance of the solution was determined by an UV−vis spectrophotometer after filtration. The residual concentrations of PAHs were determined based on the calibration curves of naphthalene, anthracene, fluoranthene, and pyrene.
Scheme 1. Synthesis of PAA Vesicles and Their Applications in Water Remediationa
a
(a) Synthesis of PAA homopolymer and its self-assembly into vesicles without purification of homopolymer. (b) Applications of PAA vesicles as a multifunctional adsorbent for the removal of both organic/ inorganic pollutants and a supporter to in situ reduce waste precious metal ions into valuable nanoparticle catalyst for catalyzing the reduction of p-nitrophenol.
vesicle shows great potential for the efficient removal of both PAHs and heavy metal ions due to strong π−π stacking, hydrophobic effect, and electrostatic interactions. Besides, cationic dyes can be decontaminated from water with an extremely low residual concentration due to the synergistic effect between π−π stacking and electrostatic interactions. Furthermore, the pollutant-saturated PAA vesicles can be easily separated from water by precipitation induced by the partial neutralization of the negative charges on the vesicle coronas and the cationic metal ions and dyes. Moreover, the PAA 714
DOI: 10.1021/acsami.7b15242 ACS Appl. Mater. Interfaces 2018, 10, 713−722
Research Article
ACS Applied Materials & Interfaces
Zeta Potential. Zeta potential studies of PAA homopolymer vesicles and Ag@vesicles were conducted at 25 °C using a ZETASIZER Nano series instrument (Malvern Instruments). UV−vis Spectrometer. The UV−vis absorbance was recorded using a UV759S UV−vis spectrophotometer (Shanghai Precision & Scientific Instrument Co., Ltd.). Fluorescence Spectroscopy. Fluorescence experiments were carried out to monitor the π−π stacking effect between PAA vesicles and different PAHs via a Lumina fluorescence spectrometer (ThermoFisher). Transmission Electron Microscopy (TEM). All the vesicle solutions were diluted at ambient temperature. Copper grids were surfacecoated to form a thin layer of amorphous carbon. Each sample (10 μL) was then dropped onto the carbon-coated grid and dried at ambient conditions. To stain vesicles, a drop of PTA (2 wt %) solution was dropped onto a hydrophobic film (Parafilm), and then those sampleloaded grids were laid upside down on the top of the PTA solution droplet and soaked for 1 min. After that a filter paper was used to carefully blot the excess PTA solution. The grids were dried under ambient environment overnight. Imaging was performed on a JEOL JEM-2100F instrument at 200 kV equipped with a Gatan 894 Ultrascan 1k CCD camera. The Ag@vesicle was viewed by TEM without staining. Scanning Electron Microscopy (SEM). SEM was utilized to observe silver nanoparticles and the surface morphologies of Ag@vesicle. To obtain SEM images, a drop of solution was spread on a silicon wafer and left until dryness. It was coated with platinum and viewed by a FEI Quanta 200 FEG electron microscope operated at 15 kV. The images were recorded by a digital camera. Elemental Analysis. An Agilent 7700 series inductively coupled plasma mass spectrometer (ICP-MS) was used to determine the concentration of metal ions adsorbed by PAA vesicles.
2.7. Adsorption of Cationic Dyes from Polluted Water by PAA Vesicles. Different cationic dyes (e.g., Victoria blue B, methylene blue, and crystal violet) were added into DI water to give a concentration of 50 μg/mL. Then PAA vesicle solution was mixed with the dye solutions for 10 min. The concentrations of PAA vesicles and cationic dyes are 228 and 10 μg/mL, respectively, and the pH of the solution was 6.9. After filtration, the absorbance of the mixtures was determined by a UV−vis spectrophotometer. Subsequently, the concentrations of the residual dyes were calculated according to the calibration curves. 2.8. Adsorption of Heavy Metal Ions from Polluted Water by PAA Vesicles. Different heavy metal salts (NiSO4·6H2O, CuSO4· 5H2O, ZnCl2, Pb(NO3)2) were added into water to prepare stock solutions with a concentration of 10 μmol/mL. Then the solution was diluted 10-fold, and the PAA vesicle was added to give a concentration of 1.0 mg/mL. After 10 min, the solution was filtered by a nanofiltration membrane and then detected by an Agilent 7700 series inductively coupled plasma mass spectrometer (ICP-MS). 2.9. Removal of Mixed Pollutants from Water by PAA Vesicles. The stock solutions of fluoranthene and Victoria Blue B were mixed together, and then PAA vesicles were added. The final concentrations of fluoranthene, Victoria Blue B, and PAA vesicles were 5.0, 10.0, and 500 μg/mL, respectively. After 30 min, the mixture was filtered by a nanofiltration membrane and tested by a fluorescence spectrophotometer and UV−vis spectrometer. For the removal of the combination of PAH and metal ion, the stock solutions of fluoranthene and ZnCl2 were mixed, and then PAA vesicles were added. The final concentrations of fluoranthene, Zn2+, and PAA vesicles were 5.0, 207 μg/mL, and 1.0 mg/mL, respectively. After 30 min, the residual concentrations of fluoranthene and Zn2+ were determined by a fluorescence spectrophotometer and ICP-MS. 2.10. In Situ Reduction of Waste Silver Ions into Valuable Silver Nanoparticles (Ag@vesicle). The aqueous PAA vesicle solution (0.4 mg/mL; 6.0 mL) was mixed with AgNO3 solution (1.0 mg/mL) at a molar ratio of 1:2 [AgNO3/(−COOH)]. After gently stirring for 30 min in the dark at room temperature, the aqueous NaBH4 solution (3.0 mg/mL) was then added to the vesicle solution immediately. The Ag@vesicle was obtained by either dialysis or centrifugation. 2.11. Traditional Silver Nanoparticles without a Vesicle Supporter. The traditional silver nanoparticles were prepared using a chemical reduction method.38 The spherical silver hydrosols were prepared by reducing aqueous AgNO3 with sodium citrate at near boiling temperature. In a typical procedure, an aqueous solution of AgNO3 (1.0 mM) was heated to boiling temperature, and then an aqueous solution of sodium citrate (1.0 mM) was added. The solution was heated until the color was greenish yellow and then cooled to room temperature. 2.12. Reduction of p-NP Catalyzed by Ag@Vesicle. The catalytic reduction of p-NP was carried out in a quartz cuvette in the presence of Ag@vesicle and NaBH4. As control, the catalyst was replaced by PAA vesicles and Ag gel, respectively. To study the catalytic efficiency, the catalyst doses were 5.0 and 15.0 μg/mL, keeping the final concentrations of p-NP at 5.0 × 10−5 M and NaBH4 at 6.7 × 10−3 M. The stock solutions of p-NP (10 μL, 0.01 M) and NaBH4 (133 μL, 0.1 M) were added to a quartz cuvette one after the other. At this stage, the n-nitrophenol was converted to nnitrophenolate anion. After that, the precalculated volumes of catalyst and DI water were added to keep the volume of the mixture to 2.0 mL. The reaction was monitored at 400 nm by the UV−vis spectrometer at different time intervals. The Ag@vesicle could be recycled by highspeed centrifugation after the reduction reaction. In order to reduce the influence of the loss of Ag@vesicle during the recycling process, the volume of the reaction solution increased to 20 mL but with the same concentrations of different additives. 2.13. Characterization. DLS. DLS analysis was conducted to determine the hydrodynamic diameter (Dh) and PD of PAA vesicles using a ZETASIZER Nano series instrument (Malvern Instruments)
3. RESULTS AND DISCUSSION 3.1. Synthesis and Self-Assembly of PAA Homopolymer. The detailed synthetic and self-assembly procedures as well as characterizations of PAA homopolymer were discussed in our previous work.37 PAA homopolymer can be massively synthesized at mild conditions and self-assembled by simply adding water into the polymer solution (Scheme 1). The critical vesiculation concentration (CVC) is as low as 0.1 μg/ mL. Besides, PAA vesicles show good stability at a broad range of pHs (from 3 to 12, Figure S1 in the Supporting Information), which makes it possible to apply the PAA vesicles at different circumstances. 3.2. TEM Analysis of PAA Vesicles. To investigate their vesicular structure, PAA vesicles were frozen by liquid nitrogen and freeze-dried under vacuum, which maintained the morphology of vesicles in solution. As shown in Figure 1A
Figure 1. (A) and (B) TEM image of PAA vesicles with different magnifications. The red arrow in (B) highlights the stack-up vesicle on the other five vesicles. The black circles are membranes stained by PTA, and the bright areas are hollow cavities of PAA vesicles. 715
DOI: 10.1021/acsami.7b15242 ACS Appl. Mater. Interfaces 2018, 10, 713−722
Research Article
ACS Applied Materials & Interfaces
precipitation of the PAA vesicles from water after 12 h when the concentration of MgSO4 is 1.0 M (Figure 2C). The reason for the salt-induced precipitation is that the added Mg2+ ions interact with carboxyl groups on the vesicle surface and bridge more vesicles together as more Mg2+ ions are added in the vesicle solution (Figure 2D). It is noteworthy that positively charged ionic pollutants in the PAA vesicles can be separated from water by the Mg2+-induced precipitation. 3.4. Ultrafast Removal of PAHs from Water by PAA Vesicles. Although the PAHs have low solubility in water, they are highly bioaccumulative and may cause mutation or cancer.41,42 The removal of trace of PAHs from aqueous solution remains an important technical challenge. Since the backbone of PAA homopolymer consists of aromatic rings, PAA vesicles can effectively adsorb PAHs from water based on π−π stacking.29 As shown in Table 1 and Figure 3, typical PAHs including naphthalene, anthracene, fluoranthene, and pyrene are used to investigate the adsorption capability of PAA vesicles.
and B, the stack-up vesicle (highlighted by the red arrow) clearly demonstrates the classical vesicular structure after staining by PTA solution. The diameter of PAA vesicles is 185 ± 29 nm by TEM analysis. The membrane thickness of the PAA vesicle is calculated to be 5 nm by analyzing the electron transmittance diagram combined with the mathematical modeling as reported previously by our group.37,39,40 3.3. Control of the Size of PAA Vesicles by Initial PAA Concentrations in DMF and the Aggregation Behavior at Different Ion Strength. The size of PAA vesicles can be easily controlled by tuning the initial concentration (Ci) of PAA homopolymer in DMF (Figure 2A). When the Ci’s increase
Table 1. Adsorption Capabilities and Residual Concentrations of Several PAHs in Water after Treatment with PAA Vesicles anthracene
fluoranthene
pyrene
35
29
5.8
0.41
3328
41
0.85
0.60
naphthalene adsorption capacity (mg/g) residual concentrations (ppb)
As shown in Figure 3A, the adsorption capacity of naphthalene reaches 35 mg/g, but its residual concentration is also as high as 3.33 ppm after filtered by a nanofiltration membrane even after 24 h (the calibration curves of PAHs are presented in Figure S2). We ascribe the poor adsorption efficiency to the weak π−π stacking between naphthalene and benzene rings. In principle, the adsorption rate and efficiency can be significantly improved if the π−π interaction between the PAHs and vesicles is enhanced. To verify the above hypothesis, anthracene, fluoranthene, and pyrene were tested, and we found that the fluorescence could be quenched within several minutes due to π−π stacking and excellent water dispersity of PAA vesicles. The residual concentration of anthracene reduces to 41 ppb with an adsorption efficiency of 99.2% (Figure 3B). In order to investigate the influence of the solid content on the adsorption efficiency, the adsorption experiments with different water volumes were carried out, as shown in Figure S3 in the Supporting Information. The results manifest that the water volume hardly affects the adsorption efficiency. As shown in Figure 3C, the adsorption capacity of fluoranthene is 5.8 mg/g with the residual concentration reducing for more than 3 orders of magnitude (>99.91%) to 0.85 ppb. For pyrene, the adsorption efficiency is also as high as 99.1%, and the residual concentration is only 0.6 ppb (Figure 3D). 3.5. Effective Adsorption of Cationic Dyes from Water by PAA Vesicles. Many dyes with different chemical structures are used in the textile industry during fiber bleaching and dyeing processes.43 Some industrial dyes are harmful to humans, especially for the lifelong pollutants with aromatic moieties due to the difficulty in the degradation.44 Therefore, different absorbents have been developed for the removal of dyes.44,45
Figure 2. Control of the size of PAA vesicles by the initial concentration of PAA in DMF and separation of PAA vesicles from water by Mg2+-induced precipitation. (A) DLS studies of PAA vesicle at different initial concentrations by directly adding water into polymer solution. (B) Size and size distribution of PAA vesicles at different concentration of Mg2+ in the solution after 30 min. (C) PAA vesicles (a) before and (b) after treated with 1.0 M MgSO4 for 12 h. (D) Schematic illustration of the aggregation of PAA vesicles induced by Mg2+.
from 1.0 to 8.0 mg/mL, the Dh’s of the PAA vesicles increase from 261 to 714 nm gradually, while the corresponding PDs of vesicles remain low. For all the samples, the preparation and characterization conditions are the same except that the initial concentration of PAA homopolymer in DMF is different. During the self-assembly process, the volume ratio of DMF to water is 1:2, and all the DMF is removed by dialysis in DI water before conducting DLS analysis. The vesicles with various sizes can meet different demands in real world applications. Furthermore, the aggregation of PAA vesicles can be easily tuned by the concentration of the divalent cation in solution. For example, when the initial concentration of the PAA homopolymer is 6.0 mg/mL in DMF, the Dh of the PAA vesicle is 530 nm with good size distribution, as confirmed by DLS studies (Figure 2A). Without the addition of MgSO4, the size and size distribution are quite stable, as shown in Figure 2B. By contrast, the diameter of PAA vesicle dramatically increases to 969 nm in the presence of 0.05 M MgSO4. The Dh further increases with the addition of MgSO4, leading to the 716
DOI: 10.1021/acsami.7b15242 ACS Appl. Mater. Interfaces 2018, 10, 713−722
Research Article
ACS Applied Materials & Interfaces
Figure 3. (A) UV−vis absorption of naphthalene solution at different time with the addition of PAA vesicles. (B), (C), and (D) Fluorescence intensities of aqueous solutions of residual anthracene, fluoranthene, and pyrene, respectively. The adsorption time is 1 min.
The adsorption efficiency of PAA vesicles for cationic dyes was investigated based on several typical dyes (Victoria blue B, methylene blue, and crystal violet). The ionic strength strongly influences the adsorption efficiency of cationic dyes because of the competitive effects, especially divalent ions, so the adsorption experiments were conducted in the absence of other cationic ions. The calibration curves are shown in Figure S4 in the Supporting Information. Figure 4 illustrates the detailed adsorption efficiency of PAA vesicles for (A) Victoria blue B, (B) methylene blue, (C) crystal violet, and (D) crystal violet. The concentration of vesicles in (A−C), (a) and (b) in (E) and (F) is 228 μg/mL, while it is 720 μg/mL in (D), (c) in (E), and (F). The concentration of dyes is 10 μg/mL, and the pH is 6.9. For Victoria blue B, after adsorption for 10 min, the UV−vis spectrum of the mixed solution was measured by an UV−vis spectrometer after filtration by a nanofiltration membrane. Notably, there are no signals between 500 and 700 nm, indicating the ultrahigh adsorption efficiency of Victoria blue B, which was also confirmed by the digital photo inserted in Figure 4A. Then the filtrate was concentrated for 100 times and measured by an UV−vis spectrometer again, but there were still no signals. Considering that the resolution of the UV−vis spectrometer is 0.005, the residual concentration of Victoria blue B should be less than 0.54 ppb, which is much lower than previously reported values.43,44,46−49 For methylene blue, PAA vesicles also show excellent adsorption efficiency (Figure 4B). The residual concentration of methylene blue is less than 0.31 ppb as determined by an UV−vis spectrometer using the same method as for Victoria blue B. For crystal violet, the adsorption efficiency is not as good as that for the above-mentioned dyes when a small amount of PAA vesicle is used (Figure 4C), which is because the electropositivity of the ammonium ion is less than that of
Figure 4. Adsorption efficiencies of (A) Victoria blue B, (B) methylene blue, (C) and (D) crystal violet with different vesicle concentrations at 10 min by PAA vesicles, and (E) and (F) corresponding photos of different dye solutions before/after adsorbed for 24 h. The concentration of dyes is 10 μg/mL. The concentration of PAA vesicle is 228 μg/mL in (A), (B), (C) and (a) and (b) in (E) and (F), while it is 720 μg/mL in (D) and (c) in (E) and (F).
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DOI: 10.1021/acsami.7b15242 ACS Appl. Mater. Interfaces 2018, 10, 713−722
Research Article
ACS Applied Materials & Interfaces
Figure 5. Concentrations of heavy metal pollutants before and after adsorbed by PAA vesicles. (A) Nickel sulfate, (B) copper sulfate, (C) zinc chloride, and (D) lead nitrate. Insets: plots on a logarithmic scale. The concentration of PAA vesicle is 1.0 mg/mL, and the pH is 6.5.
coupled plasma mass spectrometer to determine the residual concentration of metal ions. For Ni2+, PAA vesicles show excellent adsorption efficiency (Figure 5A). After filtration, the concentration of Ni2+ is reduced from 58.7 to 0.095 ppm with an efficiency of 99.84%, which is far below the national discharged standard of sewage of China (GB 20426-2006, below 0.5 ppm). For Cu2+, the residual concentration is 2.47 ppm, which is not as good as Ni2+, but the adsorption efficiency is still as high as 96.11% (Figure 5B). For Zn2+, the concentration is reduced from 65.4 to 0.58 ppm after filtration, corresponding to an adsorption efficiency of 99.11% (Figure 5C), which also meets the national discharge standard of China (GB 20426-2006, below 2.0 ppm). Pb2+ is also tested to verify the generality of PAA vesicle as an excellent adsorbent for heavy metal pollutants. After filtration, nearly 99.58% of Pb2+ is adsorbed by a PAA vesicle with a residual concentration of 0.87 ppm (Figure 5D), which is lower than the permitted concentration (1.0 ppm) of the national discharge standard of China (GB 20426-2006). As expected, the adsorption efficiencies of different heavy metal ions strongly depend on the pH of the solution, which are hard to reach 50% when the pH of the solution is 5.0 (Figure S6 in the Supporting Information).50,51 Most importantly, the desorption efficiencies of different heavy metal ions are also as high as above 80%, which guarantees the recycling of PAA vesicles (Figure S7 in the Supporting Information). It is noteworthy that the maximal adsorption capacities of the PAA vesicle to different heavy metal ions are as high as 58.6, 61.1, 64.8, and 206.3 mg/g for Ni2+, Cu2+, Zn2+, and Pb2+, which are better than most of the current adsorbents.19,20,22,23 3.7. Removal of Mixed Pollutants from Water by PAA Vesicles. It is very important for an adsorbent to possess the capability of adsorbing different pollutants simultaneously since there are several contaminants coexisting in the polluted water. In this study, different combinations of water pollutants (e.g.,
Victoria blue B and methylene blue due to the electro-donating property of the triphenylamine moiety in the crystal violet molecule. However, when the concentration of the PAA vesicle increases to 720 μg/mL, nearly all the crystal violet can be removed from water as detected by UV−vis spectroscopy (Figure 4D). The residual concentration of crystal violet is
